Effect of Fast Cooling Rate on the Microstructure and Mechanical Properties of Low-Carbon High-Strength Steel Annealed in the Intercritical Region

The effect of fast cooling rate on the microstructure and mechanical properties of low-carbon high-strength steel annealed in the intercritical region was investigated using a Gleeble 1500 thermomechanical simulator and a continuous annealing thermomeehanical simulator. The results showed that the microstructure consisted of ferrite and bainite as the main phases with a small amount of retained austenite and martensite islands at cooling rate of 5 and 50 ℃/s, respectively. Fast cooling after continuous annealing affected all constituents of the microstructure. The mechanical properties were improved considerably. Ultimate tensile strength （U-TS） increased and total elongation （TEL） decreased with increasing cooling rate in all specimens. The specimen 1 at a cooling rate of 5 ℃/s exhibited the maximum TEL and UTSxTEL （20% and 27 200 MPa%, respectively） because of the competition between weakening by presence of the retained austenite plus the carbon indigence by carbide precipitation, and strengthening by martensitic islands and precipitation. The maximum UTS and YS （1 450 and 951 MPa, respectively） were obtained for specimen 2 at a cooling rate of 50 ℃/s. This is attributed to the effect of dispersion strengthening of finer martensite islands and the effect of precipitation strengthening of carbide precipitates.

On the Microstructural Strengthening and Toughening of Heat-Affected Zone in a Low-Carbon High-Strength Cu-Bearing Steel

In this article,the influence of simulated thermal cycles for the heat-aff ected zone(HAZ)on the microstructural evolution and mechanical properties in a low-carbon high-strength Cu-bearing steel was investigated by microstructural characterization and mechanical tests.The results showed that the microstructure of the coarse-grained heat-aff ected zone(CGHAZ)and the fine-grained heat-aff ected zone(FGHAZ)was mainly comprised of lath martensite,and a mixed microstructure consisting of intercritical ferrite,tempered martensite and retained austenite occurred in the intercritically heat-aff ected zone(ICHAZ)and the subcritically heat-aff ected zone(SCHAZ).Also,8–11%retained austenite and more or less Cu precipitates were observed in the simulated HAZs except for CGHAZ.Charpy impact test indicated that the optimum toughness was obtained in FGHAZ,which was not only associated with grain refinement,but also correlated with deformation-induced transformation of the retained austenite,variant confi guration as interleaved type and a relatively weak variant selection.The toughness of ICHAZ and SCHAZ exhibited a slight downtrend due to the presence of Cu precipitates.The CGHAZ has the lowest toughness in the simulated HAZs,which was attributed to grain coarsening and heavy variant selection.In addition,the contribution of Cu precipitates to yield strength in simulated HAZs was estimated based on Russell–Brown model.It demonstrated an inverse variation trend to toughness.